The present invention relates to a process for preparing formamides and formic esters, i.e. formamide and its N-substituted derivatives and also esters of formic acid and alcohols, starting from carbon dioxide and hydrogen.
Formamide and its N-substituted derivatives are important selective solvents and extractants because of their polarity. They are used, for example, for the extraction of butadiene from C4 fractions, of acetylene from C2 cracking fractions and of aromatics from aliphatics.
Formic esters such as methyl formate or ethyl formate are used as foaming agents or fragrances.
All industrial production processes for preparing formamide and its alkyl derivatives and also formic esters have hitherto utilized carbon monoxide as C1 building block.
Formamide, N-alkylformamides and N,N-dialkylformamides are prepared by reacting methyl formate, which can be obtained by reaction of carbon monoxide with methanol, with ammonia, N-alkylamines and N,N-dialkylamines, respectively. The methanol liberated here can be recirculated to the synthesis of methyl formate from carbon monoxide and methanol. Other formic esters are obtained by reacting formic acid with the corresponding alcohol with elimination of water.
In a further synthesis which is likewise used in industry, ammonia or the abovementioned amines are reacted at from 20 to 100° C. and from 2 to 10 MPa directly with carbon monoxide instead of methyl formate. The reaction is carried out in methanol as solvent using alkoxides as catalysts (Hans-Jürgen Arpe, Industrielle Organische Chemie, 6th edition, 2007, pages 48 to 49).
Carbon monoxide is a comparatively expensive C1 building block. A further disadvantage of carbon monoxide is its toxicity, which makes it necessary to work with strict safety precautions. Furthermore, the relatively high pressures in the preparation of formamides from carbon monoxide are costly.
Numerous attempts have therefore been made to replace carbon monoxide by the cheap, nontoxic C1 building block carbon dioxide which is available in large quantities (e.g. Applied Homogeneous Catalysis with Organometallic Compounds, volume 2, pages 1058 to 1072, 1996, VCH Verlagsgesellschaft and W. Leitner, Angew. Chem. Int. Ed. Engl. 1995, 34, pages 2207 to 2221). However, most of these studies use homogeneously dissolved catalysts which are difficult to separate off from the product and recirculate to the hydrogenation.
Attempts have been made to immobilize the homogeneous catalysts on support materials so that they can be separated off more easily. This process has been described for dimethylformamide in Y. Kayaki, Y. Shimokawatoko, T. Ikariya, Adv. Synth. Catal. 2003, 345, 175-179. However, this concept requires specific support materials which are complicated to produce, e.g. diphosphine-modified polystyrenes or silica gels. Another disadvantage is that the catalyst activity decreases significantly on each recycling step. This can be attributed, inter alia, to the homogeneous catalyst not remaining fully immobilized. The preparation of the economically attractive formamide is not described in this study.
The use of heterogeneous hydrogenation catalysts can make simple isolation and reuse of the catalyst possible. U.S. Pat. No. 4,269,998 describes the use of copper chromite catalysts in combination with group VIII compounds for the preparation of dialkylformamides. A disadvantage is that only dialkylformamides can be obtained by this process. The preparation of formamide or formic esters is not described. In addition, the group VIII compounds are sensitive to carbon monoxide. However, carbon monoxide is frequently present as impurity in the hydrogen or carbon dioxide used. Thus, high-purity hydrogen and high-purity carbon dioxide have to be used in this process.
J. Liu, G. Guo, Z. Zhang, T. Jiang, H. Liu, J. Song, H. Fan, B. Han, Chem. Commun. 2010, 46, 5770-5772, describe copper/zinc oxide catalysts without a group VIII compound for preparing dimethylformamide. A disadvantage of this process is likewise that only dimethylformamide can be obtained. The preparation of formamide or formic esters is not described.